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Twin turbo SBF Drag and Drive Project

Discussion in 'The Turbo "Builds" Board' started by Wileel, May 7, 2023.

  1. Wileel

    May 6, 2023
    Im in the middle of my build and need some help selecting my turbos. Im new to the turbo world but quite familiar with all the other aspects so its been a new adventure digging into all of this.

    The car is a notchback fox, being built to run 8.50's in drag and drive events and some other local racing on a 28x10.50 radial. I'm down to selecting the turbos and getting some mixed feelings between advice and what Im seeing other people's results, Since Im putting this in the build section I'll layout what Im going with... maybe it will help someone;

    Plan is 1000-1200, not sure of the boost needed but should be 20 ish im guessing. E85 for the track and pump gas for the street. We don't have much E85 availability around me and it can be hard to find on the drives. Boost response on the street isn't super important but I don't want to have to spool at 4k for 30 sec either, especially if I end up at high altitude.

    Engine is a 438 SBF (SCAT Rotating Assy, Dart Block, Diamond pistons, 265 TFS CNC'd highport heads, port matched Holley highram intake, ect.). Engine isn't done yet so don't have compression or cam specs yet (that's another long story).

    TH400 with brake

    No converter yet...still waiting on engine specs

    Rear is QP 9" with 35 spline with a 3.00 gear

    Willwood rear and SN95 front brakes

    Magna Fuel 750 pump

    Terminator X and Leash relay board

    Headers have 1 7/8 primaries into 3" collector

    Lots more junk but not important here. Feel free to ask tho...

    Here is where I would like some input...turbos. I've decided I want run twins, and really want to run mirror image, far fewer options but I prefer the packaging and looks. Ive been told and after the research it seems a 69/73 would be the sweet spot for my set-up, but Ive heard from 66's all the way to 78/75's, even match bot gives me stuff in the upper size... this crap gets confusing lol

    Price is always a concern, and Im not really trying to make a ton of power so the units from Pulsar and VSR have been really appealing, hard to ignore compared to $2-3k for a unit from Precision . The reviews seems damn good so Im leaning this way. The 78/75 from VSR are cheap...are they way too big for me?

    Any advice or recommendations of units? And since I'm always trying to learn please tell me why you recommend what you do.

    Thanks in advance!
  2. nxcoupe

    Jan 10, 2008
    @Ben he'll jump in with knowledge. What you proposed should work well. Primaries and hotside diameter seems large to me but I'm far from a turbo expert. I just put it to the floor and let the brake go. Lol.
    silver82GT and Wileel like this.
  3. silver82GT

    Sep 20, 2005
    Cleetus is running a pair of 7675s on that Godzilla. It is about the same CID and I believe is making about 1200 hp. They seem to light off pretty easy too. YMMV.
  4. Wileel

    May 6, 2023
    Yeah, Garrets godzilla is pretty close in CID and I think mine will flow air a bit better. That's one of the builds Ive seen that seems to work pretty well and I dont see any horrible lag issues but I think they are running a smaller A/R like .83...doesnt that help keep gas velocity for spool easier?
    Also from what I gather from reading and tests like Holdner does bigger (with-in reason) has the benefit of pushing the tq load a bit further into the RPM band making internals a tad happier. Damn turbo wizardry is a deeeeeep rabbit hole
  5. silver82GT

    Sep 20, 2005
    I think you are right about the housing size, they have boost creep issues. The biggest thing about spooling a turbo is having the right converter. Forcefed86 can speak to that since he uses a pair of 7875s on a 5.3 and spools them fine.
  6. B E N

    Nov 22, 2016
    I was thinking 69-72mm range, you could probably run something as small as S366's and make your power goals. I am not real familiar with the mirror image stuff, but if you size the compressor wheel somewhere in there and don't choke the exhaust side you should be fine.

    I like the 69/73 for budget turbos for this. For a drag car I don't think the 78/75 are "too big", with a trans brake, terminator X 2 step, and your cubes they should spool fine.

    With a big engine, a good top end and a light car you aren't going to need boost to get your drag week trailer moving, or get around in traffic so you may as well go bigger on the turbos. Bigger turbos will also be less "surprising" on the street.
  7. Wileel

    May 6, 2023
    Thank you, good points! Its probably not the wisest idea to even deal with the reverse rotation/mirror image stuff, but looks great and packages a bit better IMO...just can get pricey and really limits your choices

    Precision has a twin kit they market to the LS guys in 76/75 that's not a bad deal, but both are standard.
    VSR has several that are reverse rotation but most are on what I would think are on the small side with 62 turbines, there is a 70 with the weird bullet hole anti-surge, and then the 78/75 which seems to be a deal although on the big side.

    Still waiting for any more feedback on recommendations or advise but seems lie as of now Im leaning to the VSR 78/75's.

    What would consensus be on what A/R I should use? I realize that it depends on the turbo size I select, but Im thinking over 1.25 would be too much for each and drop velocity pretty bad but only guessing here..
  8. bbi_turbos

    Dec 8, 2021
    With 1:1 drive: boost your at 39lbs/min of exhaust flow at 2.0 pr (turbine map)
    Here's a gt4294r turbine map with your point on it.. Turbine-Flow-Maps-GTX42-scaled (1).jpg #ad

    And here's your point on the same gt42 turbo compressor map. GTX4294R-Comp-Map-Kg-Sec-scaled (1).jpg #ad

    So find 2 gt4294r (70/75) with 1.01a/r housings. Not the same turbos as 78/75s, those are the older versions before they became gt42s. Much more housing/wheel choices with gt42s. Also ball bearings will help from overheating your oil, as well as require less of it. Journal bearing turbos put alot of heat and foam into your oil, as well as don't handle the g forces as well as bb.
  9. Wileel

    May 6, 2023
    Please forgive my ignorance (Im trying)...off of the graphs it seems you are showing the larger 78 is 74% efficiency and the smaller GT4294 is about 77% is that right?

    Trying to run mirrored is the real kicker, so far it seems like something will have to give, to land in a happy place. Just a cursory google shows me these but they are 73/75's (https://turbo-spec.com/products/rev...62007&pr_ref_pid=7515103592599&pr_seq=uniform)
    Looks like they are Pulsar units.

    From the research I had done it seemed that BB and billet wheels were great, but not necessarily things everyone needs. That being said I would prefer to have both if it falls into budget and/or availability. BB also have the slight advantage of spooling easier correct?
  10. bbi_turbos

    Dec 8, 2021
    I'm a dealer for pulsar turbos, they've been great, the 7375 would be as big as I would recommend going.

    Yes bb spools faster due to it having higher mechanical efficiency vs journal bearing.

    The only real benefit of billet wheel, is the wheels profile is more optimized. Cast wheels are limited by the fact that they have to be released from the mold.
    Billet can be machined with no limits, so they are more efficient.
  11. Wileel

    May 6, 2023
    Sounds very reasonable to me. I guess its subjective but, curious if twice the cost would be that much more benefit vs disadvantage... I do realize that they are both very reasonably priced. This is the hard part about learning new stuff but so far nothing but great info here so I greatly appreciate all the info. Keep it coming
  12. TwinTrouble

    Feb 4, 2003
    7675 or 7875 and never look back. I run twin gen 1 precision billet 7675’s with .96 hot sides on a 398” Windsor and they spool instantly on the brake. Even when I had a 14-45 converter. 16-0 was like a light switch. I just put in a 20-0 (about 6,000 stall since I shift at 9k) and I expect it to spool instantly.
    B E N and Wileel like this.
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